The present invention relates to a ring oscillator which is capable of measuring rising and falling edge propagation delays independently.
Known ring oscillators are used for a wide variety of applications including timing and delay propagation. When using ring oscillators to measure propagation delays with a view to characterising a circuit's performance, it is desirable to measure rising edge propagation delays independently of falling edge propagation delays.
However, most known ring oscillator frequencies depend on the sum of rising and falling edge propagation delays. In most circuits, rising edge propagation delays will be different from falling edge propagation delays. If the measurement of propagation delay is based on the average of the rising edge propagation and the falling edge propagation, as in most known ring oscillators, information regarding single edge propagation is lost.
In order to overcome the above problems, some prior art circuits have been developed which measure edge specific propagation delay. However, in order for these devices to perform the abovementioned task, a pulse must be introduced at some point in the circuit in order to set the oscillation into motion. Accordingly, these circuits must be coupled to pulse generation circuits.
The generation of pulses is a complex electronic process which requires proportionally complex circuitry. Accordingly, pulse generation circuits can be relatively large and complex. This need to couple large and complex pulse generation circuits to single edge ring oscillators has had a prohibitive effect on the integration of delay measurement circuits into very small areas of silicon.
Furthermore, with the advent of re-configurable logic fabrics, there has been an increasing desire to permit the re-configuration of selected parts of a circuit into a propagation delay characterising circuit which can dynamically be reconfigured to measure the delay of several different paths in a circuit.
Accordingly, there is a clear need for a single edge detecting ring oscillator which does not require a pulse in order to begin oscillating and which can be manufactured or configured easily and in a very small area.